Various apparatus and methods are known in the art for producing metal powders. In commonly owned U.S. Pat. No. 3,588,071 to Wentzell, apparatus is disclosed which includes a melting chamber with a crucible disposed therein, means for melting a metal placed in the crucible, means for controlling the gas pressure in the melting chamber, a vacuum chamber, disposed adjacent to the melting chamber, and an injection tube which extends from the melting chamber into the vacuum chamber for transporting the molten metal therethrough. The tube includes a plurality of holes in a portion thereof for percolating the melting chamber gas into the molten metal as it passes therethrough. This produces a metal froth which, upon discharge in the vacuum chamber, disintigrates into discrete metal particles, thereby producing a metal powder.
In utilizing the above-described apparatus, problems are encountered in controlling contamination of the metal powder. Various contaminants such as carbon, metallic vapors or ceramic particulates may be generated during the melting process and carried with the percolation gas into the froth, thereby contaminating the metal powder. Filters installed to remove the contaminants are quickly clogged, restricting gas flow and reducing powder yields.
In addition, utilizing the same gas to pressurize the melting chamber and to create the froth reduces the ability to vary powder size. As the chamber pressure increases, both the melt flow and the percolation rate increase, with the combination offsetting to produce an approximately consistent froth. To modify particle size, therefore, requires a change in the tube diameter and/or number and size of tube holes which requires substantial downtime to accomplish. Consequently, a need has arisen within the art to increase powder quality and yields, while providing increased controllability of powder size.